This invention relates to a housing for holding replaceable filters and in greater detail relates to a housing for holding multiple replaceable filters with one such filter preferably being a high efficiency particulate air (HEPA) filter used for filtering gases to remove extremely small particles, particularly small radioactive particles in an air stream, with the other filter being a preliminary filter usually replaced more frequently than the high efficiency particulate air filter.
High efficiency particulate air filters are commonly used in industry for removing particulate materials from fluid streams such as air streams, particularly when the particulate material has a high economic value or is potentially harmful when released to the atmosphere, such as a radioactive material. These filters are termed "absolute" because these filters effect a substantially total separation of particles from the gas flow, usually an air flow.
These absolute filters are customarily used by being mounted in a mounting means or a means for receiving and holding such filters inside a container or housing and capable of being replaced along with one or more other filters such as a preliminary filter mounted in a mounting means inside the housing for ease of replacement, the combination being referred to as a "filter unit." Typically the filter unit has the replaceable filters positioned in a rigid metal housing generally perpendicular to the path of flow of the fluid to be filtered through the rigid housing. The filters are sealed against a frame-shaped pressure surface located in the housing to prevent any by-pass of the filter by the flow and the filters are held in position by means of a pressure frame.
The requirements of such a filter unit that must be met during operation include stringent requirements in handling during replacement of the filters in order to avoid spreading the collected particulate material. The requirements for the safe removal from a filter housing of spent filters, and their subsequent transportation to a place of disposal, are severe. After a period of use depending upon the particular gas being filtered, its rate of flow, and other variables, the efficiency of a filter becomes reduced to a point which requires its replacement with a new element due to an increase in the pressure drop across the filter. The spent filters which are contaminated with radioactive particles give off radiation which is physiologically harmful so that it must be handled by personnel wearing protective apparel and carefully contained during and after removal from the filter housing. Likewise, the openings in the filter housing enabling loading and unloading of the filters must be enclosed or covered while unloading and loading of the filters proceed, so that no harmful radioactive material can escape to the surroundings.
It is generally the practice to prevent exposure of the spent filters or the interior of the filter housing to the atmosphere, even momentarily. In some situations it is also required that replacement filters be likewise enclosed in a container during loading into and unloading from the filter housing.